\begin{acknowledgements}
The authors thank F.~Conen and T.~Szegvary for providing 
their radon flux maps, and S.~Rast for preparing the nudging data
and making the internal review. 
We are also grateful to S.~Schery, S.~Whittlestone, C.~Schlosser and 
J.-F.~Vinuesa for their very helpful comments. 
The German BfS, French RAMCES, and Australian ANSTO monitoring networks are 
acknowledged for providing the new radon measurements used in this study.   
This work was jointly supported by the Max Planck Society and the EUCAARI project. 
All simulations were performed at the German Climate Computing Center (Deutsches Klimarechenzentrum GmbH, DKRZ). 
\end{acknowledgements}
